1. Field of the Invention
The invention relates to a new class of permanent magnet designs that can generate very strong and highly homogeneous fields for NMR, MRI and MRT use. In particular, the magnet is capable of generating field strengths of 1.0 Tesla or greater and still accommodate whole-body imaging without saturating the pole piece and surrounding magnet structures.
2. Description of the Related Art
In MRI the strength of the NMR signal is proportional to the magnetic field and so the greater the magnetic field the stronger the NMR signal which translates into improved image quality and faster acquisition of image data. For whole-body imaging field strengths greater than 0.5 Tesla were only achieved using superconducting coils typically in cylindrical configurations although recently there have been 1.0 Tesla open configurations as well. However, magnets based on superconducting coil technology require dual cryogen systems and periodic refilling making them expensive to buy and maintain. Consequently, a cheaper alternative is desirable.
As the overall MRI technology progresses demand for permanent magnet based MRI scanners has grown. Most whole-body permanent magnet based scanners are still below 0.5 Tesla and are limited in certain important applications that require greater field strengths. The limitations become severe when the required magnetic field is greater than 0.5 Tesla. First the available permanent magnets of which the strongest today is a Neodymium-Iron-Boron (NdFeB) compound can only produce a limited amount of magnetization—NdFeB compounds are capable of producing magnetic energy densities of up to 50 MGOe. Thus, there is a fundamental limit to how much magnetization these materials can produce and the magnets made from them if they are to be compact in size and weight. Second, the ferromagnetic structures and in particular the poles will saturate rendering the magnet inoperative.
A conventional magnet typically consists of permanent magnet blocks magnetized along the main magnet assembly axis which is cylindrical in configuration and are arranged to create a dipolar field with one section of the permanent magnets forming a North Pole and the other a South Pole. Each permanent magnet group has a yoke which are connected by returns and poles on the opposite sides that together concentrate the magnetic field in an air gap between the two poles where subjects are placed for the purposes of examination. In conjunction with actively shielded gradient coils and very high energy density permanent magnets such as NdFeB, with energy densities of 45 MGOe or greater, these systems can be optimized for energy efficiency to yield designs that can generate magnetic fields up to 0.55 T before becoming unwieldy in aspect ratio or size, weight and saturating the very important poles rendering the design inefficient, the field inhomogeneous, creating eddy current and residual magnetization problems and thermal drift issues.
The main source of the problem in conventional magnets is the poles. By design, these structures are used to create a constant potential surface across the pole-air gap interface whereby magnetic field lines emanating at this interface are perpendicular to the pole surface and create a highly homogeneous and energy efficient magnetic circuit. However, for typical whole-body systems, as the magnetic field strength in the air gap is required to be greater than about 0.4 T, the poles start to saturate making them ineffective or leading to designs that become impractical because they have to be made very thick. Physically, the poles introduce a boundary discontinuity and the magnetic field lines crossing this interface are forced to follow a path that is substantially curved away from the axial direction. At field strengths greater than 0.4 T, a significant part of the magnetic field energy gets concentrated in the poles saturating them and leading to an inefficient design. Moreover, the abrupt change of direction of magnetic field lines at the pole-air gap interface causes more magnetic energy leakage making the field in the air gap inhomogeneous along with many of the other problems already mentioned.
To overcome some of the problems posed by the poles more permanent magnets can be added to increase the magnetic field in the air gap. However, this solution leads to increased inefficiency because the yokes and returns have to be proportionally bigger in order to carry the additional fluxes generated that would otherwise lead to substantial leakage of energy through these structures as well.
In addition to the problems caused by the poles, the yoke and returns are further sources of magnetic energy leakage. In typical designs, there is a substantial flux path connecting the permanent magnets to the yoke and return. To minimize this leakage the returns can be placed further away from the air gap region. However, this increases the overall size, weight and footprint of the magnet.
Consequently, new methodologies are required to make stronger permanent magnet based designs. The canonical permanent magnet system to create a uniform dipolar field most efficiently is the spherical magnet with a vertically oriented uniform magnetization distribution on the surface of the sphere. However, this is a closed system and doesn't easily lend itself for MRI purposes because the subject under examination needs to be able to go in and out of the sphere. The next best solution is a cylindrical system that is infinite in length and has a vertically oriented uniform magnetization distribution on the surface of the cylinder. In practical implementations, the axial length and radial thickness of the cylinder is finite and instead of a continuous magnetization distribution an even and discrete distribution such as a Halbach array or a Magic cylinder is used.
Based on these efficient configurations, an open system can be formed by simply knocking out the middle portions of the magnetization blocks of the Halbach systems. Similar to conventional systems, rotating this system about the vertical axis will sweep out a circular geometry and make it very efficient. However, this system is obtained by breaking the original symmetry of the cylindrical and spherical systems. Therefore, one has to restore as much of the original symmetry to retain the full benefit of the canonical systems. In this invention, many features and variations on this theme have been used to compensate for the loss of symmetry of the Halbach type magnets and to design a system that is as efficient as possible.
Designs based on this approach far exceed current limits of conventional permanent magnet utility and have several novel features not found in previous MRI permanent magnet designs that simultaneously solve many of the drawbacks inherent in conventional designs.